spectrum graphics - OpenLibra

(SPECTRUM GRAPHICS!
Spirogram
Moving in circles can produce some
interesting results as Simon Palmer
demonstrates. lli&,86j I3T BlS r13
I12e,88» 137 •-IS r-id
Almost everybody will have
played with a Spirograph at
some time. It is a set of toothed
wheels, like gears which produce
intricate patterns. One of
the gears is pinned to a piece of
paper, while the other is pushed
around it by a pen which pokes
through onto the paper. The patterns
which are produced de
pend on the sizes the gears, and
the position of the pen hole in
the moving gear. These effects
are reproduced on a Spectrum
computer by this program.
The format for a call to the
machine code program is RAN
DOMIZE USR 65274: REM
a,b,f,m,r where a and b are the
coordinates of the centre of the
pattern on the screen, f and m
are integers which are the radii
of the fixed and moving gears,
and r is the distance of the pen
hole from the centre of the moving
gear.
The program is based almost
entirely on the Spectrum Basic
interpreter's calculator (called
by a RST 00281. This is controlled
by a string of literals immediately
following the call
which describe what needs to
be done, adding multiplying and
so on. Particular use has been
made of the number fetching
r IX
Stack Map
and storing facilities which are
normally used for the
calculator's memory area to
manipulate numbers on the
calculator stack (a plan of which
is given in figure 1).
Functions
The functions of the different
sections of program are:
FEF A - FF40 get the
arguments from the REM statement
which must immediately
follow a USR call to this program,
and put them on the
calculator stack after the six
dummy values at the bottom to
replace the calculator's memory
area, of at any time an error is
detected a jump is made to
1C8A which stops the BASIC
program and produces an error
report.
FF41 FF54 calculates the hcf
of f and m.
FF55 - FF74 calculates the
size of angular incrementsAG
and the total number of them in a
complete pattern.
FF75 FF8A sets the initial
set-mem get-mem contents
CO EO calcO
CI El calc 1
C2 E2 calc2
C3 E3 calc3
C4 E4 caic4
C5 E5 calc 5
C6 E6 a
CI E7 b
C8 E8 1
C9 E9 m
CA EA r
CB EB t
CC EC AG
CD ED cose
CE EE SIN e
CF EF cos AO
DO FO sin AG
D7 F1 cose
D2 F2 SIN e
D3 F3 cos Ae
D4 F4 sin Ae
D5 F5 X
D6 F6 V
< , 6 6 )
I 8 8 )
values of COS©, SIN©, COS0,
SIN0(COS & SIN denote (f i m)
cos0& (f + m) sind, and COS0&
SINfiklenote r cos0and r sin0) and
calculates cosAft, sinA0, cosA
#and sinAtT.
FF8B FF9D increments6 (by
changing COSOand SIN6I.
FF9E FFAF increments 0 (by
changing COSffand SIN0I.
FFBO - FFBB calculates the plot
position (x,y).
m l B r 1 8
no - I S r - 1
FFBC FFE2 checks to see
whether lx,y) is on the screen, if
so it is PLOTted by a USR call to
22DC.
FFE3 - FFF2 decrements the
step count, if this is not zero and
there is no keyboard break, then
the loop is executed again.
FFF3 - FFFE resets the
calculators memory area and
clears the stack.
FFFF returns to BASIC.
10 REM SPIROGRAM
20 CLEAR 65273
30 LET P=1+PEEK 23730+256*PEEK
23731
40 GO SUB 1000
50 INPUT •(-ja;",";b;•>
m";m;• r";r
60 RANDOMIZE USR P: REM a,b,-Ff
m, r
70 GO TO 50
1000 REM poke SpiroGram
1010 LET d=p
1020 DEF FN d(dS)=(dt